Optically transmissive materials, such as glass or polymers may be used as a light guide to propagate light. A light guide often includes at least one surface adapted to receive light from a light source and an optically smooth surface for reflecting light propagating through the light guide. Common examples of light guides include optical fibers traditionally used in the data communication industry and more recently light guides used for illumination purposes. For example, U.S. Pat. No. 5,432,876 discloses one such illumination device employing light guides. In this device, light may be injected into at least one end of a light guide and allowed to exit the guide at a predetermined position or positions along the length of the guide. Light extraction structures or notches are formed in the guide. The extraction structures define first and second reflecting surfaces, which reflect in a radial direction a portion of the light propagating axially through the guide. The reflected light is directed at an angle that is less than the critical angle necessary for continued propagation along the guide according to the principle of total internal reflection. As a result, the reflected light is extracted from the guide.
Because the previously mentioned light extraction structures are formed from optically smooth surfaces, they reflect light by total internal reflection rather than by diffuse reflection. As a result, light is emitted from the light guide in a pattern dictated by the configuration and arrangement of the extraction structures along the length of the guide and in a desired radial direction.
When these radial illuminating light guides are utilized in an illumination assembly, accurate orientation of the illumination light is desired.